Win2k because:
You can make it as memory hungry or light as you wish, as you can stop ALL services...
It's *NOT* slow... I use them for years... They are FASTER than XP, whatever people may say...
It doesn't "decide" that your settings suck and replace them with it's own autoconfigurated crap...
That last thing irritated me A LOT while I was setting
up a school network... Reboot and WinXP would
change the API and gateways to it's own network
crap, that worked between the WinXP machines but
not with the manually setup Win2k and Linux servers...